1. Field of the Invention
The present invention relates to a photoelectric conversion device, a method for driving a photoelectric conversion device, a radiation imaging device, and a method for driving a radiation imaging device. The radiation imaging device is a radiation reading device that performs wavelength conversion of radiation typified by α-rays, β-rays, γ-rays, and X-rays into the sensitivity range of the photoelectric conversion device by a wavelength converter, and reads information based on the radiation.
2. Description of the Related Art
In the photoelectric conversion device and the radiation imaging device, a charge based on input information arising from photoelectric conversion by a photoelectric converter is transferred to external capacitance, and is converted to a signal voltage by the external capacitance. By transferring a charge from the capacitance of the photoelectric converter itself to the external capacitance and converting it to a signal voltage in this manner, a comparatively-high S/N ratio can be obtained.
In the case of employing a configuration in which plural pixels are arranged in rows, parasitic capacitance is often formed because the line length of a signal line to which a signal is read out from the pixels increases depending on the number of pixels. For example, suppose that 2000×2000 (vertically×horizontally) pixels each having a size of 200 μm×200 μm are arranged to fabricate an area sensor having a size equivalent to that of an X-ray film, e.g. a size of 40 cm×40 cm.
In the case of the area sensor having a size equivalent to that of an X-ray film, capacitance is formed by overlapping between the gate electrode of the transistor for charge transfer and the source region thereof. This overlapping depends on the number of pixels. Therefore, even when the capacitance Cgs by the overlapping is about 0.05 pF per one place, capacitance of 0.05 pF×2000=100 pF is formed in one signal line.
The capacitance Cs of the photoelectric converter itself (sensor capacitance) is about 1 pF. Thus, if the signal voltage generated in the pixel is defined as V1, the output voltage V0 of the signal line is represented by the following equation:V0={Cs/(Cs+Cgs×1000)}×V1Therefore, the output voltage becomes about 1/100. That is, the output voltage is decreased to a large extent if an area sensor having a large area is formed.
Furthermore, to read a moving image under such circumstances, such sensitivity and high-speed operation performance as to enable reading of 30 or more pictures per one second are further required. It is also required that the dose of X-ray irradiation is as low as possible in e.g. a nondestructive test including X-ray diagnosis in a medical scene particularly. Therefore, increase in the amount of signal charge by a factor of 100 to 400, i.e. further enhancement in the sensitivity, is desired.
On the other hand, as a related art, there has been employed a configuration including, in each pixel, a source follower circuit that has a field effect transistor for receiving a signal charge generated in a photoelectric converter by its gate and reads out a signal voltage dependent on the signal charge to a signal line by this field effect transistor (refer to e.g. Japanese Patent Laid-open No. Hei 11-307756 (hereinafter Patent Document 1, refer to paragraphs 0038 and 0039 and so on particularly)). This source follower circuit enables high-speed signal readout even if the capacitance formed in the signal line is high.
However, the source follower circuit has a problem that variation in its offset potential appears as fixed pattern noise. In particular, in the source follower circuit whose semiconductor layer is composed of amorphous silicon or polycrystalline silicon, the variation in the offset potential is as very large as about 1 V.
FIG. 13 shows a related-art pixel circuit including the source follower circuit. A unit pixel 100 of the related-art example includes a photoelectric conversion element 101, a reset transistor 102, a readout transistor 103, and a row selection transistor 104.
The photoelectric conversion element 101 has the anode connected to an accumulation node N and generates a signal charge dependent on incident light. A capacitive component 105 exists at the accumulation node N, and the signal charge generated in the photoelectric conversion element 101 is accumulated in the accumulation node N. The reset transistor 102 is connected between the accumulation node N and a reference potential Vref, and resets the accumulation node N in response to a reset signal Vrst.
The gate of the readout transistor 103 is connected to the accumulation node N, and the drain thereof is connected to a power supply VDD. The readout transistor 103 receives the signal charge generated in the photoelectric conversion element 101 by its gate and outputs a signal voltage dependent on the signal charge. The row selection transistor 104 is connected between the source of the readout transistor 103 and a signal line 110, and outputs a signal output from the readout transistor 103 to the signal line 110 in response to a row scan signal Vread.
A constant current source 120 is connected to one end of the signal line 110. In this configuration, a source follower circuit is formed by the readout transistor 103 and the constant current source 120 connected to the source of the readout transistor 103 via the row selection transistor 104 and the signal line 110. The signal read out to the signal line 110 by the readout transistor 103 is output via an amplifier 130.
In the pixel circuit having the above-described configuration, the source potential of the readout transistor 103 included in the source follower circuit is lower than the gate input potential by the potential equivalent to the threshold voltage Vth of the readout transistor 103. Due to this lowering, variation in the offset value equivalent to the threshold voltage possessed by the source follower circuit, i.e. variation in the threshold voltage Vth of the readout transistor 103, appears as fixed pattern noise.
To address this problem, in the related art described in Patent Document 1, the output data of each pixel obtained without X-ray irradiation is stored in a memory as the offset value, and the offset value is subtracted from the output data obtained with X-ray irradiation. Thereby, variation in the offset value of the source follower circuit is corrected.